Process for cross-linking polymers containing carboxyl groups which comprises imbibing a shaped article of the polymer in a dhsocyanate



United States Patent PROCESS FOR CROSS-LINKING POLYMERS CON- TAINING CARBOXYL GROUPS WHICH COM- PRISES IMBIBING A SHAPED ARTICLE OF THE POLYMER IN A DIISOCYANATE Richard Watkin Rees, Graylyn Crest, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Filed Aug. 3, 1962, Ser. No. 214,480

13 Claims. (Cl. 260-775) This invention relates to new and useful polymeric compositions and the process of making such compositions. More particularly this invention relates to cross-linked addition polymers comprising at least 50 mole percent of an a-olefin and from 0.1 to 25 mole percent of an a,/3-unsaturated carboxylic acid which has been cross-linked with an unhindered diisocyanate.

Addition polymers of a-olefins often suffer from environmental stress-cracking and low resistance to deformation at elevated temperatures.

It is an object of this invention to form an addition polymer which is highly resistant to environmental stresscracking. It is a further object of this invention to improve the high temperature deformation resistance of olefins-a,B-unsaturated carboxylic acid copolymers. Another object is to provide a method of cross-linking an addition polymer in the presence of antioxidants.

These and other objects are'accomplished by reacting a diisocyanate with the carboxylic acid groups on the copolymer to effect cross-linking.

The cross-linked polymers of this invention have the advantage that antioxidants and many other additives can be incorporated in the polymer prior to cross-linking, and retain their elfectiveness in the final products. In contrast, antioxidants in polymers which are subjected to peroxide or irradiation cross-linking are attacked by the free radicals formed by the reaction and therefore are not effective in protecting the final product from oxidative degradation. Cross-linked polymers cannot readily be fabricated into shaped articles. With the cross-linking method of this invention a shaped article can be formed from the uncross-linked polymer, and the polymer of which such article is formed can then be cross-linked by treating, i.e., imbibing the article with a diisocyanate.

The polymers used in this invention are addition copolymers of an u-olefin and an a,fi-ethylenica1ly unsaturated carboxylic acid having from 3-8 carbon atoms. This includes monocarboxylic, dicarboxylic acids and half esters, amides, and nitriles of dicarboxylic acids.

The preferred a,B-unsaturated acids used have at least one free acid group and the structure C O OH wherein R =is selected from the group consisting of (111: ([JH: CH GOOH and 3,264,269 Patented August 2, 1966 $11: (EH: CH: C O ONHR 00 0B3, GEN

and R is selected from the group consisting of H, CH3, C2H5, --COOR3, GEN, CH COOH, CH CONHR and CH2CEN, wherein R is an alkyl containing from 1 to 10 carbon atoms and R is R or H. These a,;8unsaturated acids include acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, maleic acid, furnaric acid, monoesters of said dicarboxylic acids, such as methyl hydrogen maleate, methyl hydrogen fumarate, and ethyl hydrogen fumarate, half amides such as fi-carboxy acrylamide and half nitriles, ,B-cyano acrylic acid. The proportion of o e-unsaturated carboXylic or dicarboxylic acid may range from 0.1 mole percent to 25 mole percent of monomer in the total polymer composition. However, it is preferred that the polymer contains from 2 to 10 mole percent a,fi-tlnsaturated carboxylic or dicarboxylic acid monomer based on the total monomer contained in the copolymer.

The copolymer base need not necessarily comprise a two component polymer. Thus, although the olefin con tent of the copolymer should be at least 50 mole percent, more than one olefin can be employed to provide the hydrocarbon nature of the copolymer base. Additionally, any third copolymerizable monomer can be employed in combination with the olefin and the carboxylic acid comonomer. The scope of base copolymers usitable for use in the present invention is illustrated by the following examples: ethylene/ acrylic acid copolymers, ethylene/methacrylic acid copolymers, ethylene/itaconic acid copolymers, ethylene/methyl hydrogen maleate copolymers, ethylene/maleic acid copolymers, ethylene/ acrylic acid/methyl methacrylate copolymers, ethylene/ methacrylic acid/ethyl acrylate copolymers, ethylene/ itaconic acid/methyl methacrylate copolymers, ethylene/ methyl hydrogen maleate/ethyl acrylate copolymers, ethylene/methacrylic acid/vinyl acetate copolymers, ethylene/ acrylic acid/vinyl alcohol/copolymers, ethylene/ propyleneacrylic acid copolymers, ethylene/styrene/acrylic acid copolymers, ethylene/methacrylic acid/acrylonitrile copolymers, ethylene/fumaric acid/ vinyl methyl ether copolymers, and ethylene/vinylidene chloride/ acrylic acid copolymers.

Preferred three component copolymers are those which contain at least 50 mole percent ot-olefin having from 2 to 10 carbon atoms, from 2 to 10 mole percent c p-unsaturated carboxylic acid, and up to 48 mole percent of a monomer copolymerizable therewith which monomer contains no groups which will react with the diisocyanate nor interfere with the cross-linking reaction. Suitable third monomers for use in three component copolymers are preferably vinyl alcohol, vinyl acetate, and monomers having the structure 1'10 CHFC wherein R is selected from the group consisting of H, CH and Cl, and R is selected from the 3 group consisting of R phenyl, --CEN, OR and -COOR wherein R is an alkyl radical having up to carbon atoms.

Copolymers of ot-OlfififlS and an cap-unsaturated .car-' boxylic ,or dicarboxylic acid suitable for use in this invention can be prepared in accordance with any one of U.S. Patent 2,378,629, issued June 19, 1945, to Hanford; US. Patent 2,396,920, issued March 19, 1946, to Larson; or'U.S. Patent 2,396,677, issued March 19, 1946, to Brubaker; the disclosures of which are hereby incorporated by reference. Each of these patents discloses the direct production of copolymers which results in the formation of random copolymers. That is, the various monomer units are randomly distributed along the polymer chain.

Copolymers of OL-OiefiHS with carboxylic acids may be prepared by copolymerization of the a-olefin with an u,B-ethylenically unsaturated carb-oxylic acid derivative which subsequently or during copolymerization is reacted either, completely or in part to form the free acid. Thus hydrolysis, saponification, or pyrolysis may be employed to form an acid copolymer from an ester copolymer. The molecular weightof the copolymers useful as base resins is most suitably defined by melt index a measure of melt viscosity. The melt indexes referred to through-.

out this specification were obtained in accordance with ASTM-D l23 857T. The melt index of copolymers em-.

ployed in the formation of ionic copolymers which are useful as plastics is preferably in the range of 0.1 to 100 g./l0 min. and, more particularly, in the range of 1.0 to 20 g./10 min.

Suitable diisocyanates for use in this invention are those which contain up to twenty carbon atoms and are capable of reacting with a carboxylic acid. Therefore, any diisocyanate of up to twenty carbon atoms may be used except those in which the isocyanate group is sterically hindered from reaction or in which a highly reactive group in the diisocyanate molecule interferes with the reaction of such diisocyanate with a carboxylic acid.

Preferred diisocyanates for use in this invention have the structure, O=C=N-R N=C=O, wherein R is selected from the group consisting of alkyl chains having from 1 to 10 carbon atoms in the chain in which one alpha carbon atom has'the structure wherein R is selected from the group consisting of H, alkyl chains having up to 10 carbon atoms and the phenyl radical, all remaining carbons being in the form of CH aryl radicals, cycloalkanes, dipheny-l methane, bis-phenyl radicals, dicyclohexane, and biscyclo'hexyl methane. operable but not preferred due to their intense aroma.

Such diisocyanates include bis-para(isocyanophenyl) methane, better known as 4,4'-methylene-bis-pheny.l isocyanate,

H H H H H H H H toluene diisocyanate, m-phenylene diisocyanate, and 1,3 propane diisocyanate and 1,6-hexamethylene diisocyanate.

The chemical cross-linking of this invention takes place readily and is easily controlled by means of any one of or a combination of time, temperature, and amount of diisocyanate, used. This case of cross-linking control is in The corresponding thio-diisocyanates are also 4. contrast to the usual free radical cross-linking which is diflrcult to control. The chemical reaction is as follows: -CCCG-C- The following examplesv are illustrative of the inven 10111 EXAMPLE 1 Four grams of a copolymer .of ethyleneand methacrylic'acid, containing 7.2 mole percent methacrylic acid 1 and 92.8 mole percentethylene and having a melt index of 25, was dissolved in 12 cc. of'xylene. This solution was heated to C. and 2 got toluene diisocyanate wereadded. The solutionwas maintained at 100 C.

for 1 hour during which time the solution had gelled.

The product .was recovered by precipitation with methanol. The product had a melt index of 0.084, showing that cross-linking had occurred.

In Examples 3-42 .films of theethylenec p-unsaturated carboxylic :acid were prepared by compression molding; These films were 10 mils thick and were cut into a sample 6" by 6= which was then placed in a.

beaker containing 500 cc. of the undiluted cross-linking diisocyanate, at. the temperature and for thetime indicated for each example. 1

Table I Resin Cross-linking Conditions Initial Ex. Melt Isocyanate Used Tem- Melt urn-unsaturated Other monomers in resin and mole Index of pera- Time, Index of carboxylic acid and percent thereof Resin ture, seconds Crossmole percent thereof C. linked Resin 3-. 6% Itaconic acid 94% Ethylene..- 9.0 m-Phenylene diisocyanate 80 300 4 d do 9.0 Toluene diisocyanate- 80 300 0 014 5- 3.6% Fumaric acid 96.4% Ethylen 7. 8 m-Phenylene d11socyanate-. 80 300 0.16 6 0 do 7.8 Toluene diisocyanate. 80 300 3,30 7 %ldethacrylic Vinyl acetate, 70% Ethy 9.0 m-Phenylene diisocyanate 80 300 0 ac: 8 do do 9.0 Toluene diisoeyanate. 80 300 0 9 5% Methacrylic acid 95% Ethylene 6. 7 m-Phenylene diisocyanate" 80 300 0 10.-.- do do 6. 7 Toluene diisocyanate. 80 300 0 11..-. 18% 1(\1/Iethacrylic 82% Ethylene. 6.3 m-Phenylene dllsocyanate 80 300 0. 085

am 1' n do 6. 3 Toluene diisocyanate. 80 300 0 13..-- 10% l(\1/Iethacrylic 90% Ethylene 5. 8 4,4methylenebis-pheny1isocyanate 80 10 5. 02

5.8 l. -d0 80 60 3.2 5.8 (10 80 300 0.04 5. 8 nl-Phenylene diisocyanate 80 10 0. 64

5.8 o 80 60 0 5.8 .do 80 300 0 5. 8 Dichlorotoluene diisocyanate. 80 10 5 3 5. 8 d0 80 60 0, 84 5.8 d0 80 300 0 5.8 Hexamethylene diisocyanate- 80 10 0, 37

5.8 do 80 60 0 5.8 80 300 0 5. 8 Toluene diisocyanate 600 0. 9t

5.8 (10 50 00 0.007 5.8 do 60 60 0 5. 8 .d0- 60 0. 007 5. 8 .-.(10 60 15 0.015 5. 8 d0 65 15 0.010 5. 8 do 70 60 0 5. 8 do 70 15 0.0015 5.8 .do 80 60 0 5.8 ..do. 80 10 0.0033 -do 5.8 H.110 80 15 0.0032

50% Ethylene, Propylene. m-m-Biphcnyl ML... 80 800 0. 00 85% Ethylene, 5% Vinylidene chloride Bis,4-cyclohexylmethane diisocy 100 300 0.00 do 80% Ethylene, 10% Vinyl alcohol a-Methyl-hexamethylene diisocyanate 80 300 0.001

8% Methacrylie acid. 77% Ethylene, 15% Styrene 1,3 propane dilsocyanate 80 300 0. 02 40.--. 20% Itaconic acid- 70% Ethylene, 10% Acrylonitrile a-Oetyl-hexamethylene diisocyana 80 300 0.00 41--.. 7% Fumaric acid 63% Ethylene, 30% decene 1,4 butyl thio-diisocyanate 100 300 0. 001 42.--- 1% Methacrylic acid. 84% Ethylene, 15% Vinyl propiona 1,4 cyelohexane diisocyanate 80 300 0. 00

The stress crack resistance of the cross-linked compositions of this invention was tested in a solution of alkyl aryl polyethylene glycol in butanol and water at 50 C. in accordance with A.S.T.M. D-1693-60T. In this test standard Bell Telephone Laboratorys notched samples were used. In the first test the notched samples 45 were made of a copolymer containing 10 mole percent methacrylic acid and 90 mole percent ethylene which had a melt index of 5.8. Ten of these samples were cross-linked in a bath of toluene diisocyanate by heating the bath at 110 C. for 15 minutes. The ten crosslinked samples and ten non-cross-linked samples were then conditioned at 100 C. for 24 hours. The samples were then tested as above described. All of the noncross-linkcd samples failed within 24 hours. None of the cross-linked samples had failed after 800 hours in the test. Some more of the same notched test samples used above were cross-linked with m-phenylene diisocyanate at 80 C. for 15 minutes. These samples were then conditioned at 70 C. for 7 days, along with ten non-crosslinked control examples. The cross-linked samples all withstood 200 hours of the above-described stress-crack resistance test without a failure while 5 out of 10 of the control samples failed within 15 minutes. By comparison a sample of a conventional free radical polyethylene having an initial melt index of 3.70 which had been peroxide treated to yield a cross-linked polymer having a melt index of 1.40 withstood only 1% hours before 50% of the samples failed in the above-described stress-crack resistance test. Another sample of a conventional free radical polyethylene having a melt index of 2.20 was peroxide treated to yield a cross-linked polymer having a melt index of 1.82 which withstood 216 hours in the above-described stress-crack resistance test before 50 percent of the samples failed.

This invention is particularly useful in cross-linking the surface of articles formed from a-olefin/a,/3-unsaturated carboxylic acid copolymers. In particular wire jackets may be treated in accordance with this invention whereby their stress-crack resistance, and heat distortion resistance may be greatly increased. Example 43 is illustrative of cross-linking the surface of the coating on wire.

Example 43.One hundred feet of N0. 22 copper Wire was coated with 15 mils of a copolymer of 10 mole percent methyl methacrylate and mole percent ethylene in a conventional cross head wire coating apparatus. The thus coated wire was passed through a bath of toluene diisocyanate maintained at 90 C. The period of residence of each portion of wire in the toluene diisocyanate bath was 60 seconds.

I claim:

1. A process of cross-linking a shaped article formed of a random addition copolymer consisting essentially of from 0.1 to 25 mole percent of an cap-unsaturated carboxylic acid of the structure R2 Ri=( 1 COOH wherein R is selected from the group consisting of (2112:, CH=, OH: CH=,

0H3 COOH COONHR-i 7 of the copolymer being formed of vinyl acetate, and monomers having therstructure Ilia CH2=C|1 Rs wherein ,R is selected from the group consisting of H, -CH3, and Cl, and R is selected from the group consisting of R phenyl, CEN, OR'], and

COOR wherein R7 is an alkyl radical having up; to carbon atoms; which comprises the step of imbibing said shaped article with a diisocyanate free of steric hindrance, and containing up to carbon atoms, at a temperature of from C. to the melt temperature of the copolymer whereby the copolymer is cross-linked,

and the shaped article retains substantially its original shape.

2. A process of cross-linking a shaped article formed of a random addition copolymer consisting essentially of from 0.1 to 25 mole precent of an a,B-unsaturated carboxylic acid of the structure R2 Rr=( 3 coon wherein R is selected from the group consisting of 011:, 011:, CH=, CH=, CH: CH; COOH oooNHRi COORgaud (IJEN and R is selected from the group consisting of -H, CH3, C2H5, COORa, CEN, CH COOH, CH CONHR4, and -CH CEN, wherein R is an alkyl containing from 1 to 10 carbon atoms, and R is selected from the group consisting of --R and H; at least 50 mole percent of an a-olefin containto 10 carbon atoms; which comprises the step of im- 7 bibing said shaped article with a diisocyanate having the selected from the group consisting of alkyl chains having from 1 to 10 carbon atoms in the chain in which one alpha carbon atom has the structure ane; bis-phenyl radicals; dicyclohexane; and biscyclohex yl methane; at a temperature of from 25 C. to the melt temperature of copolymer whereby the copolymer is cross-linked and the shaped article retains substantially its original shape.

3. The process of claim 2 wherein R is CH 4. The process of claim 3 wherein the oc-OlBfiIllS eth-- y-lene.

5. A process of cross-linking a shaped article formed of a random addition copolymer consisting essentially of from 0.1 to 25 mole percent of methacrylic acid and the remainder ethylene, which comprises the step of imbibing said shaped article with a diisocyanate having the structure O=C=NR N=C=O, wherein R is selected from the group consisting of alkyl chains having OR7, and i from 1 to 10 carbon atoms in the chain in-which one alpha-carbon atom has structure wherein R is selected from the group consisting of -H; alkyl chains having vup to 10 carbon atoms and the phenyl radical, tall remainingcarbons being in the form of CH aryl radicals; cycloalkanes; diphenyl methane;

bis-phny1 radicals; dicyolohexane; and biscyclohexane; at-a temperature of from 25 C. to the melt temperature of the copolymer whereby the, copolymer is cross-linked andthe shapedarticle retains substantially its original shape..

6. A process of cross-linking a shaped article formed of a random addition copolyjmer consisting essentially of from 0.1 to 25 mole percent acrylic acid and the remainder ethylene,vwhich,- comprises the step of imbibing said shaped article with a diisocyanate having the selected from the group consisting of alkyl chains having from 1 to 10 carbon atoms in the; chain in which one alpha-carbon atom has the structure 3 wherein -R is selected from the; group consisting of H, alkyl chains having up to l0carbon atoms, and the phenyl radical, all remaining carbons being .in the form of CH aryl radicals; cyoloalkanes; diphenyl meth-.

vacetate, which comprises the step of imbibing said shaped article with a diisocyanate having the structure O=&N-R N=C=O, wherein R3 is selected, from the group consisting of alkyl chains having from 1 to 10 carbon atoms in the chain in which one alphacarbon atom has the structure Ra I Elf- 1 wherein R9 is selected from the group consisting of H, alkyl chain having up to 10 carbons, ,and the phenyl radical, all remaining carbons being in the form of CH aryl radicals; cycloalkanes; diphenyl methane; at a temperature of from 25 C. to the melt temperature of the copolymer whereby the copolymer is cross-linked and the shaped article retains substantially its original shape.

8. A process of cross-linking va shaped article formed of a random addition copolymer consisting essentially of from 0.1 to 25 mole percent of methacrylic acid, at least 50 mole percent ethylene, and the remainder methyl metha-crylate, which comprises the step of, imbibing said shaped article with a diisocyanate having thestructure O=C=NR N=@O, wherein -R -is selected from the group consisting of alkyl chains having from 1 to 10 carbon atoms in the chain in which one alphacarbon atom has the structure wherein R9 is selected from the group consisting of -H, alkyl chains having up to 10 carbon atoms v and thephenyl radical, all remaining carbons being in the .form of CH aryl radicals; cycloalkanes; diphenyl methane; bisphenyl radicals; dicyc-lohexane; and biscyclohexane; ata temperature of from 25 C. to the melt temperature 9 of the copolymer whereby the copolymer is cross-linked and the shaped article retains substantial-1y its original shape.

9. A process of cross-linking a shaped article formed of a random addition copolymer consisting essentially of from 0.1 to 25 mole percent of acrylic acid, at least 50 mole percent ethylene, and the remainder ethyl acrylate, which comprises the step of imbibing said shaped article with a diisocyanate having the structure wherein --R is selected from the group consisting of alkyl chains having from 1 to 10 carbon atoms in the chain in which one alpha-carbon atom has the structure and the shaped article retains substantially its original shape.

10. The process of claim 1 wherein the shaped article is a film.

11. The process of claim 1 wherein the shaped article is a wire jacket.

12. The .process of claim 6 wherein the shaped article is a film.

13. The process of claim 6 wherein the shaped article is a wire jacket.

References Cited by the Examiner UNITED STATES PATENTS 2,845,413 7/1858 Goodrich et a1. 260-94.9 2,920,067 1/ 1960 Mortimer 260'88.1 2,929,800 3/1960 Hill 26077.5 3,071,565 1/1963 Davis et a1 260-881 FOREIGN PATENTS 1,255,077 1/ 1961 France.

JOSEPH L. SCHOFER, Primary Examiner.

JOSEPH R. LIBERMAN, Examiner.

W. HOOVER, W. G. GOODSON, Assistant Examiners. 

1. A PROCESS OF CROSS-LINKING A SHAPED ARTICLE FORMED OF A RANDOM ADDITION COPOLYMER CONSISTING ESSENTIALLY OF FROM 0.1 TO 25 MOLE PERCENT OF AN A,B-UNSATURATED CARBOXYLIC ACID OF THE STRUCTURE 